The present invention relates to improvements in signal processing systems and more particularly to improved techniques and devices for providing decorrelated samples from a multiple signal environment.
Generally, signal processing systems are designed to reduce the presence of undesired signals in a signal receiving system. As is known, in particular systems, such as a radar system, the characteristics of the receiving antenna are such that undesired signals which are received in the antenna side-lobes interfere with the isolation of the target signal received in the main lobe. Accordingly, to isolate the main lobe signals, side-lobe cancellers using adaptive canceller loops have been used to cancel interference from the sidelobes of the main radar antenna as exampled by Paul W. Howells U.S. Pat. No. 3,202,990.
Conventionally, receiving systems employing adaptive loops have used auxiliary receivers to sample the environment in which a desired signal may be present. Each auxiliary signal forms an input which is used by an adaptive canceller loop in an attempt to eliminate undesired (interference) signals which may be present in conjunction with the desired signal in a main channel. Each adaptive loop attempts to decorrelate the main channel signal from its auxiliary input signal by first phase shifting and amplitude weighting the auxiliary input and then subtracting this modified auxiliary signal from the main channel.
Such systems as described above, work well to eliminate main channel interference with a single adaptive loop when only one interference source is present. However, when multiple interference sources are involved, conventional systems have been ineffective in reducing cancellation even though multiple adaptive loops have been used. In theory, if N independent interference sources are present in a signal environment, the interference signals may be cancelled from the desired signal by multiple adaptive loops fed by input from N separate auxiliary receivers. In practice, however, it has been found that effective cancellation cannot be obtained unless the auxiliary signal inputs are relatively independent of one another, in order to prevent the reintroduction of cancelled signals common in conventional systems. In addition, conventional systems using parallel connected adaptive loops have exhibited instabilities at the gains required for effective cancellation, and insufficient cancellation when constructed and operated over practically obtainable dynamic range
In copending U.S. applications 499,260 and 499,374 entitled "IMPROVED SIDELOBE CANCELLER SYSTEM" to Bernard L. Lewis and Irwin D. Olin and "MODIFIED SIDELOBE CANCELLER SYSTEM" to Bernard L. Lewis, filed in the United States on Aug. 16, 1974, and assigned to the same assignee as the present application, iterative systems are disclosed which increase the cancellation over that of conventional systems. These techniques, however, require many iterative loops, with each loop requiring multiple adaptive loops, before complete cancellation can be obtained.
In a copending U.S. application No. 499,357 entitled "ADAPTIVE SIDELOBE CANCELLER SYSTEM" to Bernard L. Lewis, filed in the United States on Aug. 16, 1974, and assigned to the same assignee as the present application, a system is disclosed which preprocesses the auxiliary signals to provide a plurality of independent samples which are then used in the main channel canceller system to cancel interference. The system as disclosed, however, requires that each auxiliary signal be decorrelated from every other auxiliary signal to form the independent samples. Since a single adaptive canceller loop can only decorrelate its output from one other auxiliary channel, multiple sequences of adaptive canceller loops connected in a series or parallel configuration are necessary before complete cancellation can be obtained. In addition, repeated iterations in series configurations are necessary before the output becomes independent of the particular sequence of inputs used, while a parallel configuration suffers from the typical problems of loop instability and restricted data input.
Accordingly, the present invention has been developed to overcome the specific shortcomings of the above known and similar techniques and to provide decorrelated input samples, particularly for use in a side-lobe canceller system.